[Y]ou take a cocktail recipe that you enjoy, add the amounts of all the ingredients, and add 1/3 of that amount in bottled water. Pour it into a bottle, refridgerate, and when guests come over you can simply pour straight from the bottle into chilled cocktail glasses. If the drink contains fruit juice, shake the bottle before serving.
I've seen the water fraction (which you add bc of the lost dilution by ice, of course) as low as 1/4, but as with most things you must fiddle to see what you think. I've talked about bottled drinks with a few bartenders, and they all admit that they do it quite regularly for catered events; the apprentices at Tales also devote a lot of time to batch cocktail production.
It's a tricky project, though, as ratios can get thrown off, citrus can grow dull, and the like. I've had success with Jack Roses, and I'm thinking about a spiced Daiquiri made with pineapple-infused rum for an event this weekend. I'd be interested to hear what sorts of drinks work well when bottled, and which drinks bomb.